Conventionally, in a tightening operation of a fastener such as a bolt, nut, or screw, an electric rotating tool such as an impact driver or a driver drill is used. In the impact driver, an electric motor is provided, and a power transmission shaft which decelerates and, at the same time, transmits the rotating force of the electric motor is provided. Furthermore, an impact mechanism including a hammer is provided on the power transmission shaft. The power transmission shaft is coupled with an anvil via the impact mechanism. When an operation that the hammer and the anvil are engaged with each other in a striking manner is repeated, intermittent rotary striking force is transmitted to a tip tool such as a driver bit held by the anvil. In the case in which a tightening operation of a screw or the like is to be carried out by using such an impact driver, when strong load torque is applied to the driver bit, the coupling between the power transmission shaft and the anvil and the driver bit is temporarily interrupted by the function of a clutch mechanism which is operated by the fastening torque that is set in advance, and the electric motor idles.
The driver drill has two operation modes which are a driver mode for carrying out a tightening operation of a fastener and a drill mode for carrying out a cutting operation of a metal plate, etc. When a fastening operation of a fastener is to be carried out by using the driver drill, the operation mode of the driver drill is set to the driver mode. In the driver mode, a clutch mechanism including a torque adjustment mechanism is disposed between a rotating shaft of the electric motor and a spindle. When the load of the driver attached to the spindle becomes larger than the rotary torque, the rotation transmission from the electric motor to the spindle is interrupted by the clutch function.
Meanwhile, when a cutting operation of carrying out boring of metal or the like is to be carried out by using the driver drill as an electric rotating tool, the operation mode of the driver drill is set to the drill mode. In the drill mode, the rotating shaft of the electric motor is connected to the anvil or the spindle without the interposition of the clutch mechanism. In this manner, in the driver drill, generally, the operation mode is switched to the driver mode (clutch mode) when the tightening operation of a fastener is to be carried out, and the operation mode is switched to the drill mode when the cutting operation of a metal plate or the like is to be carried out.